Structural Symmetry and Mixed‐anion Engineering in Halopnictides for Extraordinary Second‐Harmonic Generation

Abstract Achieving high nonlinear optical (NLO) coefficients in NLO materials remains a significant challenge. Herein, based on the [Cd 4 P 2 ] flexible host pore framework, through the structural symmetry and mixed‐anion engineering, different tetrahedral units as the guest are selected as structural directing agents for embedding, resulting in host‐guest halopnictides [Cd 4 P 2 ][ZnCl 4 ] (space group P 2 1 , 1 ), [Cd 4 P 2 ][MnCl 4 ] ( P 2 1 2 1 2 1 , 2 ), [Cd 4 P 2 ][ZnBr 4 ] ( Pna 2 1 , 3 ), [Cd 4 P 2 ][Mn 0.6 Cd 0.4 Br 4 ] ( Pna 2 1 , 4 ), and [Cd 4 P 2 ][ZnCl 3 I] ( Pna 2 1 , 5 ). Notably, we enhance sharply second‐harmonic generation (SHG) responses (0∼6.6 × AgGaS 2 @1700 nm) by synergistically disrupting local symmetry through host framework porosity tuning (54.1% to 63.1%) and constructing polarizable polyhedra via mixed‐anion modulations. Furthermore, millimeter‐sized single‐crystal (3.0 × 5.0 × 8.0 mm 3 ) 3 is successfully obtained, and possesses a strong SHG response (2.5 × AgGaS 2 ), a high laser‐induced damage threshold (3.0 × AgGaS 2 @1064 nm), and broad infrared transmittance (2.5–14.9 µm), indicating the possible promise as a NLO material. This work offers valuable insights for the development of NLO materials through combined structural symmetry and mixed‐anion engineering.